It's the weight that's required to make the spring compress. 10kgf/mm is equal to 559lbs./ft of force required to make the spring compress. An 8kg is equal to 448lbs. of force. Google "spring rates" and you'll get a better understanding. On any 5th or 6th gen Civic ('92-'00) you always want the stiffer spring up front as so the car won't sag lopsided from the engine weight at the front. Also the suspension is built around a stiffer front from the factory. Just as an FYI, Skunk2 (and Skunkworks) sleeved coilovers like you have came for Civic mad with a 500lbs. front and 400lbs. rear spring rates, so roughly 10kg and 8kg respectively minus about 50lbs which should really make them 9 and 7kg.